1. Field of the Invention
The present invention relates to a ridge waveguide distributed feedback (DFB) laser having a grating included in its ridge waveguide.
2. Background Art
Conventional ridge waveguide distributed feedback lasers using an n-type semiconductor substrate include an n-type first clad layer, an n-type second clad layer, an n-type light trap layer, a quantum well active layer, a p-type light trap layer, and a p-type clad layer formed successively on the n-type semiconductor substrate, the p-type clad layer being topped with a ridge waveguide. The ridge waveguide is made up illustratively of a p-type InGaAsP grating layer formed on a p-type InP layer, the p-type InGaAsP grating layer being topped with another p-type InP layer.
FIG. 12 shows a typical relationship between series resistance values and current values of this kind of conventional ridge waveguide distributed feedback laser. The current values are those of an oscillating current that flows to an oscillating region, arid the resistance values are those of a resistor inserted in series with the oscillating current. This kind of conventional laser has a series resistance as high as 14 xcexa9 relative to a current value of 20 mA. The reasons for this characteristic are explained below.
In such a conventional ridge waveguide distributed feedback laser, the ridge waveguide is constituted by forming a p-type InP layer topped with a p-type InGaAsP grating layer followed by burying growth of a p-type InP layer. During the burying growth, silicon (Si) piles up on regrown interfaces. The piled-up silicon, derived from the atmosphere or from water, behaves as an n-type dopant in p-type semiconductor layers. If the silicon pileup forms an n-type layer between the p-type InGaAsP grating layer on the one hand and the upper and lower p-type InP layers on the other hand, the interposed substance acts as a pnp junction conducive to higher resistance. This generally causes zinc (Zn) to be used as a p-type dopant in the p-type InGaAsP grating layer and the upper and lower p-type InP layers and to diffuse therefrom. The diffused zinc combines with the piled-up silicon and compensates the latter. As a result, the silicon no longer behaves as the n-type dopant.
In the laser structure described, the p-type InGaAsP grating layer is located immediately under the regrown interface. With this kind of conventional laser, the carrier density in the p-type InGaAsP grating layer is as low as 1xc3x971018 cmxe2x88x923 or less. That means the InGaAsP layer has a higher level of solid solubility of zinc than does the InP layer. This tends to cause zinc to diffuse from the p-type InP layer above the p-type InGaAsP grating layer into the InGaAsP grating layer, reducing the density of zinc immediately above the grating layer. As a result, the silicon pileup just above the p-type InGaAsP grating layer stays uncompensated by zinc; the remaining n-type layer then increases the resistance of the laser.
It is therefore an object of the present invention to overcome the above and other deficiencies of the prior art and to provide a ridge waveguide type distributed feedback laser which compensates piled-up silicon immediately above grating layers with zinc so as to reduce resistance of the laser significantly.
According to one embodiment of the present invention, a ridge waveguide type distributed feedback laser comprises a ridge waveguide, and a p-type InGaAsP grating layer having a carrier density ranging from 1.5xc3x971018 cmxe2x88x923 to 4.0xc3x971018 cmxe2x88x923 formed in the ridge waveguide. The p-type InGaAsP grating layer preferably has a carrier density ranging from 2.0xc3x971018 cmxe2x88x923 to 3.0xc3x971018 cmxe2x88x923.
In other embodiment of the present invention, the ridge waveguide type distributed feedback laser further comprises a p-type contact layer, and a p-type InP layer having a carrier density ranging from 1.5xc3x971018 cmxe2x88x923 to 4.0xc3x971018 cmxe2x88x923 formed interposingly between the p-type InGaAsP grating layer and the p-type contact layer.
The ridge waveguide type distributed feedback laser may comprises a quantum well active layer, and a p-type InP layer having a carrier density ranging from 1.5xc3x971018 cmxe2x88x923 to 4.0xc3x971018 cmxe2x88x923 formed interposingly between the p-type InGaAsP grating layer and the quantum well active layer.
The ridge waveguide type distributed feedback laser may comprises an upper and a lower p-type InP layer formed in a manner sandwiching the p-type InGaAsP grating layer therebetween, each of the upper and the lower p-type InP layers having a carrier density ranging from 1.5xc3x971018 cmxe2x88x923 to 4.0xc3x971018 cmxe2x88x923.
The p-type InGaAsP grating layer preferably has a carrier density ranging from 2.0xc3x971018 cmxe2x88x923 to 3.0xc3x971018 cmxe2x88x923, and the p-type InP layer preferably has a carrier density ranging from 2.0xc3x971018 cmxe2x88x923 to 3.0xc3x971018 cmxe2x88x923.
Other and further objects, features and advantages of the invention will appear more fully from the following description.